The present invention relates to the fabrication of implosion-resistant cathode ray tubes and more particularly to implosion-resistant cathode ray tube fabrication utilizing prestressed metal band strapping.
An early form of implosion-resistant cathode ray tube employed a transparent implosion plate mounted immediately adjacent or bonded to the viewing portion of the cathode ray tube envelope. Obviously, such a technique is not only expensive of components and labor but also undesirably adds weight to the structure of receiver and has a deleterious effect upon light-transmitting characteristics of the tube faceplate.
Following, it was found that a prestressed metal strapping disposed about the periphery of the viewing panel of a cathode ray tube served to provide an implosion-resistant structure. Thus, the problems of properly tensioning the metal strapping in a repetitive manufacturing operation are encountered. Moreover, the tensioning problem is increasingly complex when one considers the now prevalent substantially rectangular-shaped cathode ray tube envelope in comparison with the previously popular rounded-shaped envelope.
In one known prior technique for tightening metal strapping about the periphery of the envelope of a substantially rectangular-shaped envelope, one end of the metal strapping is firmly attached to an immovable support and the other end of the metal strapping has a force exerted thereon. Thereafter, the metal strapping is attached therebetween to provide a banded metal strapping surrounding the envelope.
Although the above-described technique was and still is utilized to provide prestressed cathode ray tube envelopes, it has been found that the technique does leave something to be desired. For example, it has been found that a force exerted on one end of the wire strapping required the strap to be drawn about three corners of the rectangular-shaped envelope. As a result, it was found that the residual tension in the strapping often varied by as much as 200-250 pounds when a comparison was made between opposite sides of the envelope. Obviously, such a residual tension variation is undesirable.
Also, the metal strapping was commonly located positionally by a chute surrounding the envelope or by manual placement. However, the chute technique resulted in the metal strapping leaving the chute upon tightening which permitted an undesired change in position with respect to the envelope. Further, the manual placement technique is dependent upon operator accuracy which is usually less reliable than a mechanical operation. Moreover, a manual operator is susceptible to injury in case of breakage of the metal strapping during application.